1. Field of the Invention
The present invention relates in general to a sensor for detecting a dynamic physical quantity such as acceleration or angular velocity. In particular, the present invention relates to a capacitance type dynamic quantity sensor for detecting a dynamic physical quantity by detecting a change in capacitance due to displacement of a structure which is manufactured through a semiconductor process.
2. Description of the Related Art
Heretofore, there has been known a capacitance type dynamic quantity sensor in which a weight adapted to be displaced due to applied acceleration or angular velocity from the outside, and beams for supporting the weight are formed within a semiconductor substrate in order to detect a change in capacitance obtained between movable electrodes of the weight and fixed electrodes formed at a minute interval from the movable electrodes (refer to JP 08-094666 A for example) FIG. 7 is a schematic cross sectional view showing a structure of a conventional capacitance type dynamic quantity sensor. In this sensor, a weight 71 and beams 72 are formed within a semiconductor substrate 73 through a fine patterning process, and are joined and sealed from both sides with upper and lower substrates 74 and 75. Such a capacitance type sensor has sensor sensitivity sensitive to minute gaps 76 and 77, and hence the fluctuation in minute gaps exerts a large influence on the sensor sensitivity characteristics. The minute gaps 76 and 77 are determined based on the gaps which are formed by selectively etching away the upper and lower substrates 74 and 75, or the semiconductor substrate 73, and the thicknesses of upper and lower fixed electrodes 78 which are formed on inner surfaces of the upper and lower substrates 74 and 75 facing the semiconductor substrate 73. Consequently, it can be said that the control for the thicknesses of the fixed electrodes is important. For example, the generation of hillocks on the surfaces of the fixed electrodes causes the fluctuation in the minute gaps 76 and 77, and also narrows a movable range of the weight to cause reduction of the sensitivity.
In the manufacturing method of the capacitance type dynamic quantity sensor disclosed in JP 08-094666 A, a glass substrate is used for each of the upper and lower substrates. Then, in order to avoid the generation of the hillocks on the surfaces of the fixed electrodes formed on the respective glass substrates, there is adopted a two-layer electrode structure having an Al layer and a Ti layer as a base layer. However, the adoption of the multilayer electrode structure increases the manufacturing cost. Moreover, the film peeling of the two-layer electrode becomes liable to occur due to a difference between the thermal expansion coefficients of the different kinds of electrode layers, reducing reliability of the sensor. Moreover, since a resistance value of the Ti layer is high, the Al layer needs to be thickened to reduce the resistance value of the total fixed electrode. As a result, the total fixed electrode becomes thick, and hence the fluctuation in the electrode thicknesses becomes liable to occur.